IN A YEAR scarred by one extreme weather event after the next, the question of how far climate change is to blame has continually come to the fore. On August 23rd a group of climate scientists issued its preliminary assessment of the role of climate change in the devastating floods that killed more than 220 people in Germany and Belgium last month. The researchers estimate that global warming has made such extraordinary rainfall up to nine times as likely as in the pre-industrial age.
Between July 12th and 15th a slow-moving low-pressure system, named “Bernd” by meteorologists, poured heavy rain onto Belgium, Germany, Luxembourg and the Netherlands. The images of widespread destruction served as a stark reminder that even the richest nations were unprepared for such unusual events.
But what role might climate change have played? Climate-modelling studies had already warned that western and central Europe would experience more pluvial and river flooding as accumulating greenhouse gases continue to push global average temperatures upwards. This was confirmed in the latest report of the Intergovernmental Panel on Climate Change, published earlier this month.
The climate scientists behind Monday’s report—organised by the World Weather Attribution group, a research coalition—used climate models to compare the July floods with what might have happened in a hypothetical world in which average temperatures were the same as before the industrial revolution. Similar studies have been used to assess other recent extreme-weather events. They have concluded that a Siberian heatwave in 2020 was made as much as many thousand times more likely by human greenhouse-gas emissions, and that the heatwave in the Pacific Northwest earlier this year would have been virtually impossible in the absence of global warming.
Such “climate attribution” studies are far easier to carry out for anything related to extreme heat, because there is a direct and relatively simple relationship between global warming and air temperature. Anything involving water, however, is more complex. A hotter atmosphere can hold more moisture and so deliver fiercer downpours, but these also depend on local and regional weather conditions and, crucially, floods are not created just by rainfall. They also depend on a great number of topological and geological factors, such as the depth and saturation of soil, the shape of the landscape and the capacity of waterways. The same amount of water will have very different consequences if it falls on a flat plain or in a long, narrow mountain gully.
In mid-July, unusually heavy rain fell on top of already wet terrain, making the flooding more severe. Around the Ahr and Erft rivers in Germany, for instance, 93mm bucketed down in a single day; in Belgium, around the Meuse, 106mm fell in two days.
The team modelled the odds of this kind of precipitation falling in different areas (130km by 130km for the purposes of the study) between the north of the Alps and the Netherlands, and ran the simulations under two different scenarios: one in which the world was 1.2°C warmer than in pre-industrial times, as it is today, and one in which the world had not warmed at all.
They found that climate change made the kind of extreme rainfall recorded in July between 1.2 and nine times as likely as in the hypothetical unwarmed world. (The wide range reflects the breadth of results produced by different climate models.) The modelling also found that climate change has increased the amount of rain that can fall in one day in Western Europe by 3-19%. Running the models in another hypothetical world that is 0.8°C warmer than today’s (or 2°C warmer than in pre-industrial times) increased the chances of an event further still, by 1.2-1.4 times.
Even in today’s climate, the heavy rainfall last month was “a very rare event”, says Maarten van Aalst of the Red Cross Red Crescent Climate Centre, who contributed to the study. The team estimates that locations in western Europe can expect similar rainfall only once every 400 years.
That is less reassuring than it sounds. The study shows, in accordance with the IPCC report, that the risk of these rare extreme events is rising and will become greater still as the world continues to warm. What is more, if different regions in Western Europe are each likely to suffer such events only once every four centuries, they are likely to happen somewhere in the whole continent much more often.
Climate change means that societies need to re-think how they prepare for rare but devastating events, says Mr van Aalst. “We thought our experience from the past could help us prepare for the worst case scenario that you might get in the future. That is no longer the case.” More and worse extremes of all types, he adds, “is something we need to rationally plan for rather than wait until experience has taught us, because then it’s often too late.”
Image credit: AFP